Proposed patch adds the const qualifier to char* parameter declarations and static char arrays if appropriate. This makes the code more strict and can help to catch bugs or generate more efficient code. In additional it can decrease memory consumption by avoiding copying constant data.

New changeset 3cb3e224b692 by Serhiy Storchaka in branch 'default':
Issue #25923: Added the const qualifier to static constant arrays.
https://hg.python.org/cpython/rev/3cb3e224b692

New changeset 80d1faa9735d by Serhiy Storchaka in branch 'default':
Issue #25923: Added more const qualifiers to signatures of static and private functions.
https://hg.python.org/cpython/rev/80d1faa9735d

What is the point of changing the return value of a function in
Objects/memoryobject.c to "const char *" and then casting it back
to "char *" in the one place that the function is used?

Churn like this has a direct cost for me: I have to update my
private 100% coverage patch for no gain at all.

get_native_fmtstr() returns a pointer to C string literal. It is constant by its nature, and I thought it would be more clear to cast it explicitly if needed. It would be better (in particular for compatibility with C++) to make Py_buffer.format to be const char*, since it is borrowed reference to constant string, and often is initialized with string literal, but this is backward incompatible change of public API. I'm planning to open separate issue for backward-incompatible (controlled with a macro) constantness changing.

I was faced with the same dilemma when I wrote the code and I
*deliberately* decided that returning (const char *) + an
additional cast wasn't worth it for a static function
that's used only once.

Sorry. Do you prefer to revert this change or left it as is?

There's no need to revert it.  Both versions are okay, but I wanted
to point out that these kinds of changes a) add a cognitive load for
other developers (I have to look what has changed in memoryobject.c)
and b) may override other developers' conscious decisions.

Excuse my troubling.